1. Field of the Invention
The invention is related to the field of communication systems, and in particular, to allocating ATM connections in ATM networks.
2. Statement of the Problem
Circuit-based networks use Time Division Multiplexing (TDM), Frequency Division Multiplexing (FDM), or other techniques to multiplex multiple calls onto a single physical link or medium. Multiplexing the calls increases the bandwidth available on the physical link. Packet-based networks, such as Asynchronous Transfer Mode (ATM) and Internet Protocol (IP) networks, also use multiplexing techniques to multiplex multiple calls onto a single physical link. For ATM networks, there may be two layers of multiplexing. One layer comprises multiplexing multiple calls on a physical link. The second layer comprises multiplexing multiple calls on a single ATM connection on the physical link.
Multiplexing calls on an ATM connection, along with using silence suppression in voice calls, results in a statistical gain in the average bandwidth needed for a call. The statistical gain depends on the number of calls to be multiplexed on the ATM connection. The statistical gain is higher when a large number of voice calls with silence suppression are multiplexed on the ATM connection, as the bandwidth per call is significantly less than would be needed without silence suppression. The statistical gain goes down as the number of voice calls multiplexed on the ATM connection goes down. As an extreme example, a single call on an ATM connection results in no statistical gain.
With packet-based networks being built, there is a need to interface the traditional circuit-based networks with the newer packet-based networks. For the interface, gateway devices are used to interwork data between the circuit-based networks and the packet-based networks. Gateway devices include media gateways and gateway controllers. For an ATM network, media gateways connect a switch in the TDM network to an ATM switch in the ATM network. The media gateways consequently convert formats between TDM format and ATM format to allow devices from the two networks to talk.
To understand the current state of the art, assume that a first gateway device, such as a media gateway, is coupled to a second gateway device in an ATM network by a DS-3 link. The first gateway device receives a call setup message for a call from the TDM network. The first gateway device processes the call setup message to determine that the call needs to be routed to the second gateway device over the DS-3 link. If there is an ATM connection already allocated between the first gateway device and the second gateway device over the DS-3 link, then the first gateway device performs a Connection Admission Control (CAC) function to determine if there is bandwidth available on the ATM connection or if there are sufficient resources to handle the call. If there is bandwidth available, then the first gateway device routes ATM cells, representing the call, to the second gateway device over the ATM connection.
If there is no ATM connection allocated between the first gateway device and the second gateway device, then the first gateway device requests an ATM connection over the DS-3 link. As part of the request for the ATM connection, the first gateway device requests a particular bandwidth for the ATM connection.
To determine the amount of bandwidth to request for the ATM connection, the first gateway device estimates the number of calls that will be routed over the ATM connection. The first gateway device also determines the average bandwidth per call, considering silence suppression, encoding, voice compression, fax calls, modem calls, etc. The first gateway device determines a low-end estimate of the needed bandwidth for the ATM connection by multiplying the number of calls and the average bandwidth per call.
The low-end estimate may not be a good estimate for the ATM connection, as bandwidth may not be reserved for up-speeding, such as for a fax or modem call. For instance, assume, in operation, that the low-end estimate is the actual bandwidth allocated to the ATM connection and the first gateway device multiplexes 24 calls on the ATM connection. If the 24th call multiplexed on the ATM connection is a fax or modem call, then there most likely will not be enough bandwidth on the ATM connection for the fax or modem call. The actual bandwidth for a fax or modem call will be much higher than the average bandwidth per call assumed for the low-end estimate. Because there is insufficient bandwidth on the ATM connection, the first gateway device will drop the fax or modem call.
To solve this problem, the first gateway device adds additional bandwidth to the low-end estimate to determine the actual bandwidth to request for the ATM connection. The additional bandwidth builds a safety factor into the actual requested bandwidth so that the ATM connection can handle fax and modem calls, or other calls that exceed the average bandwidth of a call.
Once the actual requested bandwidth is determined, the first gateway device transmits a request for an ATM connection to the second gateway device along with the actual requested bandwidth for the ATM connection. Responsive to the request, the second gateway device determines if there is bandwidth available on the DS-3 link for the ATM connection. If the requested amount of bandwidth is not available, then the second gateway device does not allocate the ATM connection for the first gateway device. If the requested amount of bandwidth is available, then the second gateway device does allocate the ATM connection for the actual requested bandwidth.
After the second gateway device allocates the ATM connection, the first gateway device routes the call over the ATM connection. If bandwidth is allocated on the ATM connection for about 24 calls, then the first gateway device may multiplex up to 24 calls on the ATM connection. The first gateway device and the second gateway device actively police the ATM connection and the DS-3 link to ensure that the first gateway device is not exceeding the bandwidth of the ATM connection or the bandwidth of the DS-3.
Unfortunately, allocating ATM connections in the manner described above may not be efficient. The statistical gain achieved in an individual ATM connection is unfortunately dependent on the number of calls assumed for that ATM connection. The more calls being routed over the ATM connection, the higher the statistical gain, and the fewer calls being routed over the ATM connection, the lower the statistical gain. In the prior art method of allocating ATM connections, the estimated average bandwidth per call used to determine the bandwidth to request for the ATM connection is dependent on the number of calls assumed for that ATM connection. It may be advantageous to allocate ATM connections where the statistical gain is not dependent on the number of calls.